The decrease in size of electronic equipment has increased the use of liquid crystal displays (hereinafter referred to as LCDs) as a display means. The LCD is not only used as the screen of a computer, but also for other displays such as television screens, screens of projectors, etc. Displays utilizing liquid crystals have low power consumption, due to their use of low drive voltages, and relatively high response speed, and because of these advantages, it is expected that their use will expand to other applications in the future.
Most of the currently used LCDs are of the active matrix type. The active matrix type is one in which a drive circuit element is built in to each pixel to improve display characteristics. Active matrix LCDs, using thin-film Three-terminal transistors (TFTs) as switching elements are referred to as TFT type liquid crystal displays.
FIG. 1 shows a schematic diagram of a pixel portion of the array of TFT-type liquid crystal display. In one pixel opening 1, there are disposed a display electrode 2, a gate line 3, a gate electrode 3A, a data line 4, a drain electrode 4A, a source electrode 5, and a TFT active element 6. When the TFT is turned on by the signal on the gate line 3, the data line 4 is connected to the display electrode 2 via the source electrode 5 to make the potential of the data line 4 equal to the potential of the pixel electrode 2. As a result, the liquid crystal sealed above the pixel electrode 2 in the direction perpendicular to the drawing sheet is oriented to put that pixel in a display state.
In the sectional view shown in FIG. 2, the TFT active element portion in FIG. 1 is shown as a TFT 10 formed on a lower glass substrate 20. The TFT 10 is separated form an upper glass substrate 21 by a predetermined distance, and liquid crystal is sealed in a space 11 between the lower glass substrate 20 and the upper glass substrate 21. The liquid crystal (not shown) varies in its orientation according to the signal applied to the display electrode formed on the lower glass substrate 20. Because no pixel electrode exists in the area where the TFT 10 is formed, the orientation of the liquid crystal there cannot be controlled. In this uncontrolled area, a light intercepting layer 12 is provided to prevent the transmission of light. The light intercepting layer 12 is usually formed of an oxide of Cr. On the rear of the light intercepting layer 12 (on the lower glass substrate side), a light intercepting film 13 is formed. For the light intercepting film 13, Cr is usually used for compatibility with the light intercepting layer 12.
A sectional view of the TFT 10 is shown in FIG. 3. As shown in this sectional view, wiring material portions such as the gate electrode 71, which are formed on an undercoat portion 72 on the glass substrate 70, are isolated by a gate insulation film 73 from transparent electrode 74, an amorphous silicon layer 75, and source/drain electrodes 76 and 77. The gate insulation film 73 is composed of a silicon oxide or a silicon nitride, and laminated on the wiring material portion. The amorphous silicon layer 75 is interposed in the source and drain regions 76, 77, and operates as the channel region of the TFT 10.
Returning to FIG. 2, color filters 14 and 15 are formed on the lower surface of the upper glass substrate 21. The color filters 14 and 15 are usually films of an organic matter colored either red, green, or blue. The color filters 14 and 15 are arranged so that the adjacent ones are different in color. As shown in FIG. 4, the TFT 10 shown in FIG. 2 exists under the color filter of blue in the left side of FIG. 4, and the light intercepting layer 12 (shown by a rectangular frame) is positioned above it. Between the color filters 14 and 15, there is a black matrix 25 which does not transmit light. Returning to FIG. 2, light in path (a) is incident from below the lower glass substrate 20, and modulated by the orientation of the liquid sealed in the liquid crystal sealing portion 11, and colored when passing through the upper glass substrate 21 having the color filter 14 formed on the surface thereof.
One of the applications of the LCD, is using the liquid crystal as light modulation means (light valve) for projecting and magnifying in an overhead projector (OHP). A schematic diagram of this application is shown in FIG. 5. A LCD module 31 is placed on an OHP apparatus 30. The OHP apparatus 30 includes a light source 36, and the light emanating from the light source 36 responses to the signal for display supplied form a control unit 35 to the liquid crystal module 31, and modulated and colored in the liquid crystal module 31. Then, it is projected onto a screen 36 by an appropriate direction control and magnifying means 32. The principle of this is well known. The application of this kind is characterized in that the light source is very strong. That is, in the conventional LCD, the light source is included in the LCD and relatively weak, and the light emanating from the light source is modulated by an electric signal according to the supplied data to display a desired pattern. However, in this application, the light emanating from the light source included in the OHP apparatus 30, which is a light source outside the LCD, is modulated and colored by the liquid module. And, the light source 36 included in the OHP apparatus 30 generally has a light intensity greater than the light source included in the LCD, and usually cannot be optimized for use with the liquid module.
If the TFT-liquid module is used as a light valve while using such strong light source 36, many blue bright spots (point defects) are observed on the display screen and as a result, display characteristics are not realized. While the cause of the problem was not clear, it was well known that such point defects occur in applications in which a strong light source is used. The applications using a strong light source include a search light, the incidence of sunlight and the like in addition to the OHP.
As the conventional solution to this problem, a method of reducing the quantity of the light arriving at the TFT has been employed. One specific means for doing this, is to laminate color filters in layers for enhancing the optical density. However, the lamination process has process stability and residuum removal problems, and in addition, introduces problems such as inappropriate cell gaping that occurs because the increased thickness of the color filter.